1. Field of the Invention
The present invention relates to a surface mount filter and, more particularly, to a surface mount filter used as a part of small equipment such as a cellular phone and a cordless telephone for radio communication using a microwave frequency band.
2. Description of the Relevant Art
A surface mount filter with the following characteristics has been widely known. Its construction comprises external I/O terminals on surfaces of a dielectric block and plural striplines formed inside the dielectric block. The space between the striplines varies along the length direction, and one end of each stripline is open while the other end is electrically connected with the external electrodes.
FIG. 1 comprise diagrammatic views of a conventional surface mount filter of the above kind (Japanese Kokai No. 05-55810), where FIG. 1(a) is a perspective diagram showing the inside of the filter and FIG. 1(b) is a perspective view showing the filter mounted on a printed circuit board.
Two dielectric blocks 41 are formed from a material having a high dielectric constant .epsilon. in the shape of a rectangular parallelepiped board, respectively, and are joined. On the top surface 41b, bottom surface 41c, rear surface 41d, and side surfaces 41e and 41f of the joined dielectric block 41, external electrodes 42b-42f made of a conductive material and input/output terminals 46a and 47a are formed.
On the top surface of the lower dielectric block 41. striplines 43 and 44 made of a conductive material are formed almost in the shape of a hook. The stripline front portions 43a and 44a are extended in parallel to the front surface 41a (open end portion) of the dielectric block 41. On the other hand, the stripline rear portions 43b and 44b are elongated in parallel and are connected to a rectangular connecting conductor 45a, which is connected to the external electrodes 42d, 42e, and 42f. Here, the relationship of a distance d.sub.1 between the stripline front portions 43a and 44a with the distance d.sub.2 between the stripline rear portions 43b and 44b is d.sub.1 &gt;d.sub.2.
The first ends of input/output lines 46b and 47b are connected to prescribed places of the striplines 43 and 44, while the second ends of the input/output lines 46b and 47b are connected to the input/output terminals 46a and 47a, respectively. Two connecting conductors 45b are formed in the shape of elongations of the external electrodes 42e and 42f in the vicinities of both left and right ends of the dielectric block 41, in parallel with the stripline front portions 43a and 44a. The lower filter 40a, includes the dielectric block 41, external electrodes 42c-42f, striplines 43 and 44, input/output terminals 46a and 47a, input/output lines 46b and 47b, and associated parts, while the upper filter 40b includes the dielectric block 41, external electrodes 42b and 42d-42f, striplines 43 and 44, input/output terminals 46a and 47a, input/output lines 46b and 47b, and associated parts. A surface mount filter 40 comprises the lower filter 40a and the upper filter 40b.
In the manufacture of the surface mount filter 40 having the above construction, a conductive paste (Ag or Cu paste) is applied to prescribed places on the lower dielectric block 41 and is baked so as to form the striplines 43 and 44, the external electrodes 42c-42f, and the like, leading to the preparation of the lower filter 40a. At the same time, the upper filter 40b, having a plane shape symmetrical to the lower filter 40a, is prepared. Then, by bonding the planes of the lower filter 40a and the upper filter 40b, on which the striplines 43 and 44 are formed, to each other, the surface mount filter 40 is manufactured.
By connecting the external electrode 42c to a ground electrode 52 formed on the surface of a printed circuit board body 51, while the input/output terminals 46a and 47a are connected to input/output signal lines 53 and 54 through connecting terminals 55, respectively, the surface mount filter 40 is mounted onto a printed circuit board 50.
FIG. 4 is a schematic circuit diagram showing an equivalent circuit for the surface mount filter 40. Inductances L.sub.1 comprise the dielectric block 41 and the stripline front portion 43a or 44a. To the first ends of the inductances L.sub.1 are connected capacitances C formed between the stripline front portion 43a or 44a and the connecting conductor 45b. The capacitances C are connected to the ground electrode 52 through the external electrodes 42e or 42f, and 42c. On the other hand the first ends of inductances L.sub.2 comprising the stripline rear portion 43b or 44b are connected to the second ends of the inductances L.sub.1, while the second ends of the inductances L.sub.2 are connected to the ground electrode 52 through the connecting conductor 45a and the external electrodes 42d, 42e or 42f, and 42c.
Between the inductances L.sub.1 and L.sub.2, the input/output terminal 46a or 47a is connected through the input/output line 46b or 47b, respectively. A quarter wavelength resonator 48 includes the stripline 43, external electrodes 42b-42e, connecting conductors 45a and 45b, input terminal 46a, and associated parts, while a quarter wavelength resonator 49 includes the stripline 44, external electrodes 42b-42d and 42f, connecting conductors 45a and 45b, output terminal 47a, and associated parts. The surface mount filter 40 includes those resonators 48 and 49.
In the surface mount filter 40 having the above construction, the inductances L.sub.1 make mutual-inductance coupling (hereinafter, referred to as M.sub.1), while the inductances L.sub.2 make mutual-inductance coupling (hereinafter, referred to as M.sub.2). The distance d.sub.2 is shorter than the distance d.sub.1, leading to M.sub.1 &lt;M.sub.2, so that the electromagnetic coupling between the resonators 48 and 49 is ensured by the balance between M.sub.1 and M.sub.2. Therefore, a filtering treatment of prescribed frequency regions is conducted on microwave signals which are input into the input terminal 46a in the resonators 48 and 49, so that only the signals within a prescribed band width are output from the output terminal 47a.
As another conventional surface mount filter different from the above-mentioned surface mount filter 40, a surface mount filter having almost the same construction as the lower filter 40a shown in FIG. 1 was proposed (Japanese Kokai No. 05-283907). provided that the filter does not include the external electrodes 42e and 42f, connecting conductors 45a and 45b, and input/output terminals 46a and 47a. Another different point from the lower filter 40a is that, as prescribed places of an external electrode 42c are notched, input/output terminals are formed on the dielectric block bottom surface 41c opposed to input/output lines 46b and 47b so as to cause the input/output terminals and the input/output lines 46b and 47b to make capacitive coupling. Here, the distance d.sub.1 between stripline front portions 43a and 44a is 5 mm or so, for example.
In the above surface mount filter 40, in order to ensure the electromagnetic coupling between the adjacent resonators 48 and 49, the relationship of the distances d.sub.1 and d.sub.2 between striplines 43 and 44 is d.sub.1 &gt;d.sub.2. Since d.sub.1 need be larger, miniaturization is difficult. In order to make filter characteristics, especially a harmonic rejection characteristic at a fundamental frequency, more favorable, M.sub.1 needs to be set smaller than M.sub.2 at a prescribed rate. Accordingly, the distance d.sub.1 needs to be larger, and so it becomes more difficult to achieve miniaturization.
Since the striplines 43 and 44, having elaborate shapes and the like, are buried inside the dielectric block 41, the filter is difficult to manufacture. It is impossible to regulate the filter characteristics by modifying the distances between the stripline front portion 43a or 44a and the connecting conductor 45b and the like after mounting the filter on the printed circuit board 50. Since the input/output terminals 46a and 47a are not formed on the bottom surface 41c of the dielectric block 41, the input/output terminals 46a and 47a cannot be directly connected to the input/output signal lines 53 and 54 on the printed circuit board 50. As a result, it requires much time to connect the input/output terminals 46a and 47a thereto.
In the above different surface mount filter, striplines 43 and 44 are in exposed positions, so that noise which comes flying from above easily enters the striplines 43 and 44. In order to inhibit the entry of the noise, it is necessary to additionally prepare a shield to cover the filter, leading to a high cost.